There are many examples where evaporation is used to reduce the liquid phase of water solutions containing contaminants for the purpose of concentrating the contaminants for disposal. Often referred to as thermal separation or thermal concentration processes, these processes generally begin with a liquid and end up with a more concentrated but still pump-able concentrate that may be subjected to further processing and/or disposal. In the context of this description, waste water solutions containing contaminants are referred to as “raw water”.
The liquid reduction requirements dictated by the physical characteristics of raw water have resulted in the development of a large range of different types of evaporators over the years. Demands for energy efficiency, minimized environmental impact, low capital cost and low operating cost have driven evaporator development toward various plant type configurations and equipment designs. In the design of evaporation systems, numerous, and sometimes contradictory requirements have to be considered, which may determine which type of construction and configuration is chosen. The resulting principles of operation and economic performance between different designs may vary greatly. By way of background, various design considerations may include:                Capacity and operational data, including quantities, concentrations, temperatures, annual operating hours, change of product, controls, automation, etc.;        Product characteristics, including heat sensitivity, viscosity and flow properties, foaming tendency, fouling and precipitation, boiling behavior, etc.;        Required operating media, such as steam, cooling water, electric power, cleaning agents, spare parts, etc.;        Capital and collateral financial costs;        Personnel costs for operation and maintenance;        Standards and conditions for manufacture delivery, acceptance, etc.;        Choice of materials of construction and surface finishes;        Site conditions, such as available space, climate (for outdoor sites), connections for energy and product, service platforms, etc.; and        Legal regulations covering safety, accident prevention, sound emissions, environmental requirements, and others, depending upon the specific project.        
Based on the above, the applications and systems for evaporative concentration of raw water are diverse requiring design decisions being based on the deployment. For example, in some deployments, it is particularly important that mobile water treatment plants are reliable and straight-forward to operate by onsite personnel.
One specific application that benefits from the use of an efficient mobile evaporative unit is the onsite processing of raw water generated on and around a drilling rig that is produced from snow or rain accumulations washing over equipment and/or other raw water produced or recovered at the drilling rig lease.
Many environmental regulations prohibit raw water to be discharged directly from the drilling lease surface area onto the surrounding ground regions due to the level of contamination that may be present in the raw water. For example, raw water may be contaminated with oils, soaps, chemicals and suspended particulates originating from the drilling rig operations.
Normally, at a drilling rig, raw water must be collected in peripheral ditches constructed as a first line environmental discharge barrier. In some cases, the volume of raw water may become sufficiently great during rig operations to inhibit the efficient operation of the drilling rig as the volume of raw water interferes with the operation and movement of equipment and personnel at the drill site. In these cases, the raw water must be collected and/or removed to permit drilling rig operations to continue.
Often, in the absence of systems allowing on-site processing, the raw water must be collected from the ditches, stored in holding tanks and eventually trucked to a remote processing center for processing and disposal. As known to those skilled in the art, the collection, storage, transportation, processing and disposal of the raw water at the remote location can be very costly both in terms of actual handling and processing costs but also from lost time at the drilling rig.
In the past, there have been systems to reduce the liquid volume of raw water by boiling off the aqueous phase of the raw water with a mobile water evaporator/boiler. One such system is a diesel fueled boiler that heats the raw water in a tank to boil raw water that may have been pre-clarified through a series of settling tanks mounted on a skid based evaporator system. The raw water is boiled in place to produce a concentrated slurry as the aqueous portion of the raw water is boiled off that settles near the bottom of an evaporator tank by gravity acting on an increasingly dense fluid. This bottom concentrate is periodically removed from the evaporator/boiler system by various systems such as vacuum suction.
There are a number of inherent problems with existing evaporator systems as listed and discussed below. These problems include:                Systems that must be operated in a batch process mode. In these systems as any new addition of raw water to the bulk storage tanks halts the evaporation process and requires reheating the whole system before vaporization can resume;        The inefficient use of heat energy due to increasingly limited thermal transfer from the heat source to the raw water, that may be caused by:                    a buildup of particulate and scale that coats various parts of the system such as a heat exchanger, promoting increasing heat loss out the heating system exhaust stack; and/or            the need to thermally heat un-separated, suspended particulates in the raw water tank as the density increases;                        Unnecessary fuel consumption, due to overall system inefficiencies. In this case, fuel consumption may have to be increased to meet target processing rates resulting in higher costs to the operator and greater volumes of combustion contaminants being discharged to the atmosphere;        Foaming and frothing of hot or boiling solutions over the sides of the tank into the surrounding environment that may be occurring in close proximity to personnel. Such problems may also require the use of anti-foaming agents and system supervision;        Frequent and time-intensive system cleaning;        Intensive and/or invasive onsite supervision to ensure the evaporator system flow dynamics are within certain narrow parameters to prevent automatic shut down and restarts;        Heating element damage from over-heating due to concentrate accumulation on a heat exchanger; and        Soaps and oils present in the raw water that may cause surface layering that inhibits the evaporation process.        
A review of the prior art reveals that contaminated water evaporators can transfer heat to the contaminated water mass using a variety of methods to reduce the volume and weight of the concentrated water for transportation and final disposal.
For example, Canadian Patent 2,531,870 issued Mar. 18, 2008 entitled “Evaporator System” and Canadian Patent 2,554,471 issued Sep. 16, 2008 entitled “Self-Powered Settling and Evaporation Tank Apparatus” exemplify the current commercial prior art of ditch water evaporators. Typically these prior art systems are batch process systems where a tank is filled with the contaminated water and a heat source is applied near the bottom of the tank to transfer the heat to the total mass of contaminated water. The heat source can be any number of heating methods such as steam, electrical resistance heaters and/or hot gasses derived from combustion or hot liquids. In these systems, the heat source must elevate the temperature of the total contaminated water mass in the tank to a level before it can begin to boil off any of the water. Generally, these systems must also reheat the water mass each time additional water is introduced into the reservoir thus significantly slowing the over-all evaporation process.
Over time, evaporation of the water from the tank with the added contaminated water increases the concentration of the non-evaporated constituents within the tank. While these systems will concentrate raw water, it should be noted that as the concentration of the solids and other contaminants in the concentrated water increases, the likelihood that more contaminants from the evaporator will be carried from the system with the evaporated water vapor also increases.
In high temperature driven evaporators, because of the high temperature differential needed to pass heat from the source through the heating element into the water, and because of the presence of chemical salts and other contaminants, the heating element is subject to scaling, fouling and corrosion. Heating element coating creates a significant decrease in efficiency within a very short time and requires frequent and intensive cleaning. Additionally, from the moment the heating element becomes coated (e.g. with scaling), which is almost instantaneous upon system start up, heat is increasingly inhibited from passing through the element into the water and thus is wasted out the flue stack. Complex control systems must sometimes be used with prior art evaporators to account for this fluctuation in exhaust gas temperature over time.
Additionally, when transferring heat through a heating element, the heating element surface area becomes key to the thermal transfer rate and efficiency. Typically the higher the evaporation rate required the more surface area is required on the heat element. Therefore these systems are not scalable on site. If they are to be scaled they must be remanufactured with different physical parameters.
Further still, in these systems, the increasing total solids mass concentration also decreases the efficiency of the evaporator due to the applied heat being absorbed by any solids in the tank. As well, such solids also tend to line the tank surface and cover the heating elements, tubes, and other components in the tank such as level sensors and other monitoring instrumentation that will affect heat transfer and the overall efficiency of operation.
Still further, another significant problem with various prior art systems is the stratification of the waste water due to any soaps or organic material that may be present in the waste water. The presence of either or both of these contaminants will often generate a surface skim or layer on top of the waste water that interrupts the water mass evaporative process. To counter this problem, some past systems incorporate significant complexities into a design to prevent and/or mitigate issues the effects of these contaminants in the water evaporation process. Moreover, soap and/or organic materials can cause significant foaming and frothing that can often result in overflowing the heating tank and spillage onto the ground requiring expensive clean-up operations and/or putting the operator at substantial environmental and safety risk.
A still further problem with various evaporators is particulate material is not removed from the raw water prior to transferring the raw water into the evaporator tank thereby resulting in the need to remove the accumulated solids frequently and/or, as noted above, the unnecessary heating of particulate matter during evaporation. Drains are typically provided in the tank to remove the sludge from the tank; however, the sludge must generally have a high water content in order to permit the sludge to flow through the drain.
Further still, sludge that remains coated on the tank and other elements requires periodic cleaning, usually with steam or water. The sludge and cleaning water, as a product of the cleaning process, must also be hauled away which increases the total cost of operating the evaporator.
Examples of past systems also include those described in U.S. Pat. Nos. 7,722,739, 5,259,931, U.S. Patent Publication No. 2009/0294074, U.S. Pat. Nos. 5,770,019, 5,573,895, 7,513,972, 2,101,112, and No. 6,200,428.
As a result, and in view of the foregoing, there has been a need for thermally efficient, continuous processes for waste water contaminant concentration that can mitigate the various problems associated with the prior art systems.
In addition, there has also been a need for a system with the capability to concentrate waste water using waste heat generated from normal drilling rig operations in order to provide further operational and efficiency advantages over systems in which a regular fuel supply is required.
Further still, there is also a need for a system that is also simultaneously effective in evaporating water and in removing combustion related soot, particulate and combustion chemicals from the heat source if applicable to the particular heating source. In other words, heretofore there has been no incentive for mobile treatment of flue gasses because there is generally no regulation on diesel engine exhaust to justify the cost of doing so. As such, and until regulation is set, the cleaning of these collectively large volumes of acid gasses will not occur. While there are clear environmental benefits to cleaning engine exhaust at a well site, within the current regulatory framework, this will occur if the technology for cleaning exhaust is part of another system. Accordingly, by marrying the technology for cleaning exhaust gasses with another use such as evaporating wastewater, there is an economic incentive to the operator to take this environmentally responsible action.
In regards to the emissions from drilling rig operations, there are generally over 2,000 rigs operating in North America with each one consuming on average approximately 3,000-9,000 liters per day of diesel fuel within the various power generating machinery. For example, for a typical 500 kW engine-generator set, each 500 kW engine, capable of evaporating over 10 cubic meters of water per day, will exhaust approximately 91-273 cubic meters per minute of acid gas exhaust into the environment thereby polluting the environment and wasting the heat energy contained therein. This equates to 95-285 billion cubic meters of uncleaned acid gas discharge from all North American rigs every year.
Thus, there has also been a need for systems that can reduce the amount of exhaust contaminants that may be released to the atmosphere while at the same time reducing the total volumes of contaminated waste water that requiring shipping and/or removal from a drilling rig site.